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inbucket
Disposable webmail server (similar to Mailinator) with built in SMTP, POP3, RESTful servers; no DB required.
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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
I like MailU (https://mailu.io). Quickly deployed with Docker too.
If all you need is a system to receive SMTP mail and read the messages via web GUI then inbucket is probably the easiest solution.
NOTE:
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Hence, a higher number means a more popular project.